Start with the sliced mushrooms. Cook them until they’ve given up nearly all their liquid. It takes longer than you think. Add the onions and cook until soft. Alternately, use dried mushrooms and add them to the onion and butter mix for microwave caramelized onions.
I thicken with a roux, so I add the butter to the mushroom onion mix, melt it, and then add an equal part flour. 2 tbsp each per cup of liquid you add. You can also thicken with cornstarch after adding the liquid.
Slowly whisk in the liquid, stirring until smooth after each addition. I like using a cup of broth. If using broth concentrate, use water and add the broth concentrate after adding all the water to the roux.
Once you have the sauce, add a tablespoon of marmite. Then add the sour cream and season to taste.
I usually cook the meat separately, e.g. broiling then slicing it. I add it at the end.